1. Field of the Invention
The present invention relates to a steering apparatus of impact energy absorbing type, which is designed to absorb impact energy applied to steering means, such as a steering wheel, from the driver when a car makes a crash or the like.
2. Description of Related Art
A steering apparatus of impact energy absorbing type comprises: an upper housing for rotatably supporting a steering shaft having an upper portion coupled to a steering wheel via an antifriction bearing; a lower housing which is inserted into the upper housing so as to be relatively movable in the axial direction and is fixed to the car body; and first and second impact energy absorbing protrusions which are projected in one body from a lower portion of the upper housing separately from each other in the axial direction so as to contact with the inner circumferential surface of the lower housing, for example.
Impact energy caused by a secondary crash, which is applied to a steering wheel when a car makes a frontal crash or the like, is transmitted through the antifriction bearing to the upper housing, so that the upper housing moves in the axial direction with respect to the lower housing. While the upper housing moves, the impact energy is absorbed by frictional resistance generated between the impact energy absorbing protrusions and the lower housing.
However, since impact energy caused by the secondary crash is applied to the steering shaft in a direction inclined with respect to the axis of the steering shaft, bending moment arises at the upper housing which supports the steering shaft. This bending moment causes the upper housing to be inclined with respect to the lower housing, so that the lower end of the upper housing comes into contact with the outer circumferential surface of the lower housing and the upper end of the lower housing comes into contact with the inner circumferential surface of the upper housing, thereby applying load due to frictional resistance to these two contact parts. As a result, movement resistance of the upper housing increases and load applied to the driver also increases.
The load applied to the driver can be reduced by lengthening the distance between the first impact energy absorbing protrusion and the second impact energy absorbing protrusion so as to reduce the above-mentioned load due to frictional resistance applied to the two contact parts. It is, however, impossible to lengthen the distance between the two impact energy absorbing protrusions since the movement amount of the upper housing, i.e. the energy absorption stroke, is inevitably determined by the layout of the vehicle.
Another steering apparatus widely known is a steering apparatus of impact energy absorbing type disclosed in Japanese Patent Application Laid-Open No. H10-76958 (1998), for example, in which a lower housing is engaged with an upper housing for rotatably supporting a steering shaft having an upper portion coupled to a steering wheel via two impact energy absorbing rings made of synthetic resin disposed separately from each other in the axial direction so as to be relatively movable in the axial direction.
The steering apparatus disclosed in Japanese Patent Application Laid-Open No. H10-76958 absorbs impact energy in a state where the circumferential surface of the impact energy absorbing rings are in contact with the circumferential surface of the upper housing and the lower housing. As a result, the load due to frictional resistance can be reduced and the movement resistance of the upper housing can be reduced in comparison with a steering apparatus having impact energy absorbing protrusions. However, since the upper housing receives bending moment corresponding to steering force in usual steering as well as bending moment in impact energy absorption, heavy load is applied to the impact energy absorbing rings. Consequently, there has been a demand for improvement in durability of the impact energy absorbing rings.